1. Technical Field
The present invention relates to convection-cooled projectors and more particularly to employing a low illuminance lamp as a light source used in conjunction with a screen with a high gain rating.
2. Description of Prior Art
Prior art projectors in general use LCD panel projectors comprises a fan cooled box-like housing which supports an illumination lamp, an LCD panel bearing an image to be projected and various configurations of lenses used to magnify the image on the LCD panel. The exhaust fan is incorporated in the housing or exhausting air inside the housing through the air outlet. An 800 Watt to 1000 Watt projection light source is also provided in the housing for illuminating LCD panel. A projection lens is provided for projecting a magnified image on the LCD panel onto a remote viewing surface such as a screen and includes a focusing lens and may include a reflecting mirror for focusing and directing the image from the LCD panel to the screen. The projector of the above-described type recently employs a high illuminance lamp, for example, a halogen lamp, as the light source so that the visibility of the enlarged image projected on the screen can be improved. In this case, however, LCD panel placed would be subjected to high energy heat rays produced by the high illuminance lamp such that the LCD panel goes above 45 degrees Centigrade at which point it ceases to function properly. The use of a low illuminance lamp in conjunction with a high gain screen eliminates the need for a cooling fan. Means need to be provided enhancing convection cooling to stop heat buildup in the projector housing.
In the conventional projectors of the transmissive LCD panel type, air heated by the heat rays from the light source is exhausted by the exhaust fan of the housing. Furthermore, a cold mirror which reflects light energy but few heat rays is employed as a spherical reflecting mirror mounted in the rear of or below the lamp. Moreover, a heat ray-reflecting filter is interposed between the lamp and the Fresnel lens.
Transmissive LCD panels are used with the above described overhead projectors, however, the LCD panels must not heat up to above 45C or they will cease to function. Some LCD projectors have built in fans to provide additional cooling in addition to the cooling fans built into the conventional projectors utilizing high luminance lamps. These LCD panels functionality is easily degraded by heat. Furthermore, dark colored portions of a projected area of the transmissive LCD panels absorb heat rays. Consequently, the above-described conventional means cannot provide sufficient prevention of temperatures above 45C if a secondary fan is not included in the transmissive LCD panel. Use of the low illuminance lamp allows the removal of the need for the secondary fan in the transmissive LCD panel. See for example U.S. Pat. Nos. 3,711,194, 4,880,303, 4,882,599, and 4,904,079.
U.S. Pat. No. 3,711,194 an overhead projection apparatus is disclosed having a housing with a lamp casing therein for projecting a light beam onto a reflective surface that directs a beam upwardly through a film onto a lens assembly for projection of an image onto a screen.
U.S. Pat. No. 4,880,303 is directed towards an overhead projector having a housing with a Fresnel lens, a lamp is positioned within the housing to provide illumination to a liquid crystal display framed in a hinge mounting on the side of the housing which can be positioned down an operative position overlying a Fresnel lens. A projection lens is positioned in spaced relation to the LCD panel and a reflective mirror surface is provided for the output of the projection lens.
U.S. Pat. No. 4,882,599 an overhead projector is illustrated having a housing configuration with an internal light source and lens plus an internal reflective mirror which reflects an image up through a liquid crystal display and then through a projection lens and an angular image mirror thereabove.
U.S. Pat. No. 4,904,709 a liquid crystal display device for overhead projector is illustrated and described in which a liquid crystal lens display is sandwiched between protective plates which are separated from the cell surface to form an air passage provided with a fan so that an airflow can be directed there through which will come in direct contact with the cell cooling same.
Finally, in U.S. Pat. No. 5,683,160 an overhead projector with light illumines lamp as a light source is disclosed in which a housing is illustrated having a light source and mirror within and a pair of air inlets and outlets formed in the respective sidewalls with an exhaust fan incorporated within the housing to provide a cooling flow of air over the projected lens surface.